While there can be many contributing factors to defects in injection molded articles, trapped gases within a mold cavity is often at least a significant cause. Part quality shortcomings that can be attributed to trapped gases include short shots, inconsistent distribution of park weight, flash (particularly in thin-walled parts), sink, air bubbles, black spots, and warpage. Trapped gases entrained in molten resin can also ignite in the mold cavity, causing charring to regions of the molded part.
Various methodologies have been employed in conventional injection molding systems to vent gases during the injection molding process. For instance, ventilation channels may be built into the walls of a mold cavity, vent plugs made of porous metals, and negative pressure or vacuum assist methodologies have been utilized. However, these methodologies require costly modifications to existing molds or customization of molds during their initial manufacture so as to provide effective venting. It would be desirable to provide an injection molding system that achieved adequate venting of gases from molten polymeric resin as the molten material is being introduced to the mold cavity, thereby reducing or eliminating the need to provide extensive venting in the mold itself.
By venting gas upstream of the mold cavity, an added advantage is that molten resin introduced to, and filling, the mold cavity has less gases entrained therein. Since gas entrained in the molten resin displaces the molten resin, such entrained gas can result in the delivery of less actual molten resin than desired.